In many data retrieval systems, especially large, multi-use database systems, there are frequently more operations submitted to the system than the system can support at one time. To improve efficiency of the data retrieval system, it is useful to impose some sort of resource scheduling criteria on the operations seeking to retrieve data from the data retrieval system. One scheduling criterion that is frequently used is execution time. Operations that are running for a long time, and consuming a lot of resources, have their access restricted. This is typically done by a human monitor or administrator of the data retrieval system, who checks the status of the operations running on the data retrieval system, and selectively de-activates or re-schedules operations that have been running for too long. Long-running operations can be re-scheduled to run at less-busy times, such as at night, or after business hours.
This procedure, however, has several drawbacks. It requires the intervention of an administrator, who must constantly monitor the status of all of the operations running on the data retrieval system. Furthermore, de-activating a long-running operation results in system resource wastage, since the partially-completed processing of the operation is frequently lost upon de-activation. Additionally, users of the data retrieval system are inconvenienced because they don't find out that their operations were consuming too many resources until after the operation has been running for a lengthy period of time.